Structure and Function of Skeletal Muscle

Let's begin by working our way down through levels of organization in a skeletal muscle until we reach the contractile units that actually generate the force. The entire muscle, as well as the individual cells, are wrapped in collagen. Near the end the collagen merges to form the tendons, which attach the muscle to the bone. It is through this connective tissue that the force generated by the individual cells is transmitted to the bone.

A group of muscle cells are bundled together by collagen to form a fascicle. Since muscle cells are elongated and cylindrical, each muscle cell is usually called a muscle fiber. In skeletal muscle, the muscle fibers are very large, multinucleated, and up to several millimeters in length.

Looking at one muscle fiber, you will see that almost the entire cross section of the muscle fiber is taken up by long, cylindrical strands of contractile proteins called myofibrils. Typically there are hundreds of these in one cross section of a muscle fiber. Looking at one myofibril, we see that it is divided into segments called sarcomeres. These are the contractile units of a muscle. A dark stripe called a Z disc marks the ends of one sarcomere and the beginning of the next.

Sarcomeres are composed of thick filaments and thin filaments. The thin filaments are attached at one end to a Z disc and extend toward the center of the sarcomere. The thick filaments, by contrast, lie at the center of the sarcomere and overlap the thin filaments.

Look at the diagram above and realize what happens as a muscle contracts. The thick and thin filaments slide with respect to one another, using ATP as a source of energy. As a result of the sliding, the Z discs are pulled closer together. This is called the sliding filament mechanism. The contraction of a whole muscle fiber results from the simultaneous contraction of all of its sarcomeres.

The contraction of a muscle fiber is triggered by an action potential conducting over plasma membrane of the muscle fiber. The action potential conducts from the surface of the muscle fiber into the interior via transverse tubules(T tubules). These long tubes are continuous with the plasma membrane.

As a T tubules passes each myofibril, it touches, but is separate from, membranous bags called the sarcoplasmic reticulum. These are wrapped around each sarcomere and are filled with Ca++.

When an action potential in a T tubule reaches each piece of sarcoplasmic reticulum, the action potential triggers the opening of Ca++ channels in the sarcoplasmic reticulum. As a result, Ca++ flows out of the sarcoplasmic reticulum and into the saromere with its thick and thin filaments. This causes the filaments to start sliding and thus the sarcomere to shorten. But very quickly, the Ca++ is actively transported back into the sarcoplasmic reticulum and the sarcomere relaxes.

The thick filaments are comprised of an elongated protein called myosin. Each myosin molecule is shaped like a golf club, with the head of the golf club pointed out from the surface of the thick filament. This structure will form the cross bridge that binds to the thin filament.

Actin is the main protein of the thin filament. A second protein, called troponin, is found at intervals. When Ca++ binds to troponin, this allows myosin heads to bind to the actin of the thin filament, creating cross bridges. The cross bridges then pull on the thin filaments, causing the sarcomere to shorten. The cross bridges then release the actin, with one molecule of ATP used by each cross bridge in each cycle. When Ca++ is present, this cycling of cross bridges continues and the filaments continue to slide with respect to one another. When Ca++ goes back into the sarcoplasmic reticulum, the contraction stops.

Next to each muscle fiber are a few small satellite cells, which retain some of the embryonic characteristics. Notably, they can fuse with damaged muscle fibers and help repair the damage.

Quick Quiz

Fill in Answer Correct False Correct Answer
What are the long strands of contractile proteins which nearly fill the cross section of a skeletal muscle fiber?
What is the entire structure that lies between two Z discs?
What protein makes up the thick filaments?
What protein binds Ca++ in skeletal muscle?
What membranous bag stores Ca++ in a skeletal muscle fiber?
What long, thin membranous structure carries the action potential to the interior of a skeletal muscle fiber?

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